Avoid unnecessary "touch meson.build" in vpath builds

Avoid unnecessary "touch meson.build" in vpath builds

In e6927270cd1 I added a 'touch meson.build' to configure.ac, to ensure
conflicts between in-tree configure based builds and meson builds are
automatically detected. Unfortunately I omitted spaces around the condition
restricting this to in-tree builds, leading to touch meson.build to also be
executed in vpath builds. While the only consequence of this buglet is an
unnecessary empty file in build directories, it seems worth backpatching.

Reported-by: Christoph Berg <myon@debian.org>
Discussion: https://postgr.es/m/20240417230002.mb2gv3hyetyn67gk@awork3.anarazel.de
Backpatch: 16-, where the meson based build was added